Not to my knowledge but there are rare buggy things that can always happen randomly with MnB2 like that Stop Fighting after giving a command/ upgrade to a soldier.
I think I had the same thing happen to a green Gunner of mine recently.
May be that when giving a soldier the Moral Boost they stop whatever they are doing and stand at attention.
Possible that that soldier gets stuck in attention mode sometimes.
Maybe-Remedy;
*Move the effected soldier around.
*Allow him to get pinned.
*Give another Moral Boost later on.
*All around defense command may make him fight again
*basically do so something to him that requires a forced reaction.
That a Tactical Retreat not working then is not know to me.
If all fails use as a meat shield or on a suicide run to make enemy cannons FF themselves.